Polymerization of conjugated dienes, especially butadiene-1,3, is well known in the art using a variety of catalysts, including a mixture of a cobalt and an aluminum compound. Thus U.S. Pat. No. 3,135,725 discloses cobalt salt-hydrocarbyl aluminum compound catalysts and U.S. Pat. No. 3,046,265 discloses alkyl aluminum, cobalt halide and acetyl halide catalyst, both catalyst systems being useful in polymerizing conjugated diolefins. The following additional cobalt-aluminum compound catalyst systems are disclosed in the indicated patents for polymerization of conjugated diolefins: cobalt chloride-methylaluminum chloride in U.S. Pat. No. 3,094,514, cobalt octoate-ethylaluminum chloride in Br. Pat. No. 916,383, cobalt ethyl hexoate-ethylaluminum chloride in Br. Pat. No. 916,384, cobalt octoate-diethylaluminum chloride and water in German Pat. No. 1,495,935 and cobalt stearate-diethylaluminum chloride in Br. Pat. No. 941,739.
Although generally the order of addition of the reactants and the catalysts apparently was not considered to be of critical importance, various patents disclose different orders of addition. Thus in British Pat. No. 926,036 the reactants and catalysts were added in the order of solvent, aluminum compound, cobalt chloride in pyridine and finally butadiene while in British Pat. No. 924,427 the order was: aromatic solvent, cobalt and then aluminum compounds as catalysts, additional aromatic solvent, aliphatic solvent and finally butadiene. U.S. Pat. No. 3,284,431 discloses adding to the reaction vessel an aromatic solvent, butadiene, an aluminum compound catalyst an activator and finally a cobalt compound catalyst. Another order of addition is disclosed in U.S. Pat. No. 3,646,001 where cobalt and aluminum catalysts, such as cobalt octoate and diethyl aluminum chloride, are prereacted in wet benzene at a temperature below 20.degree. C. and then are added butadiene and additional solvents such as benzene and butene-1. Published Japanese patent application SHO-44-10276 discloses dissolving a cobalt catalyst in dry toluene, feeding butadiene-1,3 to the reaction vessel and then adding an aluminum catalyst to begin polymerization. This method, however, yields cis-1,4-polybutadiene having a relatively high gel content.